We present a search for the e+e− decay of a hypothetical dark photon, also named U vector boson, in inclusive dielectron spectra measured by HADES in the p(3.5 GeV) + p, Nb reactions, as well as the ...Ar (1.756 GeV/u) + KCl reaction. An upper limit on the kinetic mixing parameter squared ϵ2 at 90% CL has been obtained for the mass range MU=0.02–0.55 GeV/c2 and is compared with the present world data set. For masses 0.03–0.1 GeV/c2, the limit has been lowered with respect to previous results, allowing now to exclude a large part of the parameter region favored by the muon g−2 anomaly. Furthermore, an improved upper limit on the branching ratio of 2.3×10−6 has been set on the helicity-suppressed direct decay of the eta meson, η→e+e−, at 90% CL.
The treatment of large uveal melanomas poses a therapeutic challenge, due to the expected treatment-related side-effects. After sole radiotherapy the majority of patients are faced with radiogenic ...complications secondary to the large amount of tumour necrosis. Alternative treatment modalities addressing this issue are transscleral resection in arterial hypotension in anteriorly located tumours and endoresection via pars plana vitrectomy in posteriorly located tumours. A surgical resection treatment was applied in 292 patients with large uveal melanomas. In 150 patients the tumour was treated by transscleral resection and postoperative adjuvant (106)ruthenium brachytherapy and 142 patients were treated by primary proton beam irradiation and secondary endoresection. The mean follow-up time was 3.8 and 2.5 years, respectively. Local tumour control was achieved in 76 % and 98 %, respectively. The 5-year metastatic rates were 28 % and 21 % and eye retention was achieved in 82 % and 97 %, respectively. Surgical resection of uveal melanomas with adjuvant radiotherapy is the treatment of choice in cases of large tumours, avoiding enucleation in the vast majority of cases in the long term, without increasing the incidence of tumour-related risks.
To describe the long-term tumour control and metastatic rate after transscleral resection (TSR) of large uveal melanomas in a single-centre study.
The sample included 210 patients with large uveal ...melanomas. Univariate analysis of local tumour control and metastatic risk by Kaplan-Meier survival curves and log-rank testing. Cox proportional HR analysis with forward and backward selection was used to identify independent prognostic factors in patients submitted to TSR of a large uveal melanoma.
A residual tumour was diagnosed in 6% of the patients. The 5- and 10-year local tumour recurrence rates were 24% and 32%, respectively. Older age, a large basal tumour diameter, the lack of adjuvant ruthenium brachytherapy and retinal detachment led to a 2.6, 2.4, 4.4 and 7.8 times higher risk of melanoma recurrence, respectively. The 5- and 10-year metastatic rates were 28% and 44%, respectively, and were statistical significantly affected by extraocular spread, tumour thickness and local tumour recurrence.
TSR is an alternative to enucleation for the treatment of large uveal melanomas. Results should improve with better patient selection and more effective methods of adjuvant radiotherapy.
The production of Σ0 baryons in the nuclear reaction p (3.5 GeV) + Nb (corresponding to sNN=3.18 GeV) is studied with the detector set-up HADES at GSI, Darmstadt. Σ0s were identified via the decay ...Σ0→Λγ with subsequent decays Λ→pπ− in coincidence with a e+e− pair from either external (γ→e+e−) or internal (Dalitz decay γ⁎→e+e−) gamma conversions. The differential Σ0 cross section integrated over the detector acceptance, i.e. the rapidity interval 0.5<y<1.1, has been extracted as ΔσΣ0=2.3±(0.2)stat±(−0.6+0.6)sys±(0.2)norm mb, yielding the inclusive production cross section in full phase space σΣ0total=5.8±(0.5)stat±(−1.4+1.4)sys±(0.6)norm±(1.7)extrapol mb by averaging over different extrapolation methods. The Λall/Σ0 ratio within the HADES acceptance is equal to 2.3±(0.2)stat±(−0.6+0.6)sys. The obtained rapidity and momentum distributions are compared to transport model calculations. The Σ0 yield agrees with the statistical model of particle production in nuclear reactions.
We analyse of a set of
radio rich (accompanied by type IV or II bursts) solar flares and their association with SOHO/LASCO Coronal Mass Ejections in the period 1998–2000. The intensity, impulsiveness ...and energetics of these events are investigated. We find that, on the average, flares associated both with type IIs and CMEs are more impulsive and more energetic than flares associated with type IIs only (without CME reported), as well as flares accompanied by type IV continua but not type II shocks. From the last two classes, flares with type II bursts (without CMEs reported) are the shortest in duration and the most impulsive.
We present a hypothesis that the anomaly in the folding angle distribution of electron-positron pairs, emitted in the decay of the excited levels of nucleus \(^{8}\)Be and \(^{4}\)He can be related ...to the cluster structure of the decaying state. Furthermore, we present a hypothesis that the potentially observed boson with re st mass \(m_{X}\)=17 MeV can mediate the nucleon-nucleon interaction at the low-energy regime of QCD, in particular in the weakly bound cluster state p+\(^{7}\)Li,\(^{3}\)H. We present a possible equations of state of symmetric nuclear matter corresponding to the vector meson mass \(m_{v}\)=17 MeV, obtained using relativistic mean field theory of nuclear force, QHD-I in particular, with physically relevant incompessibility K\(_{0}\)=240-260 MeV and the values of couplings \(g_{v}\), \(g_{s}\) lower than unity. Based on concepts of chiral symmetry breaking, we show that reduction of the rest mass of pseudoscalar particle from physical value \(m_{\pi}\)=135 MeV to \(m_{X}\)=17 MeV is equivalent to reduction of the quark mass from dynamical value around 310 MeV down to current quark mass around 5 MeV (\(\frac{m_{X}^2} {m_{q,curr}} \simeq \frac{m_{\pi}^2} {m_{q,dyn}}\)). Corresponding version of Goldberger-Treiman relation leads to the value of coupling close to the results from relativistic mean field theory of nuclear force. Both model approaches thus point towards apparent restoration of chiral symmetry in nucleon-nucleon interaction at large distances, possibly via bounce into false instanton vacuum. Observation of boson with rest mass \(m_{X}\)=17 MeV in the decay of high lying excited states of \(^{8}\)Be and \(^{4}\)He can possibly resolve one of the longest lasting open questions in nuclear physics.
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